The invention relates to an arrangement of an electrically coupled acoustic-wave transmitter and receiver, which arrangement serves to prevent the occurrence of oscillations as a result of acoustic feedback.
Self-sustained oscillation may for example occur in so-called "loudspeaking" telephone sets. In these sets, which are equipped with a loudspeaker and a microphone arranged comparatively close to each other, undesired oscillations may be produced in the loop comprising the acoustic path between the loudspeaker and the microphone and the electrical path if no precautions are taken. These oscillations are caused by an incorrect decoupling between the channel connected to the loudspeaker and the channel connected to the microphone. In order to preclude said oscillations, which give rise to a howling sound which renders the telephone set almost unusable, the known methods influence the electrical part of the loop. Said known methods are of two general types mentioned in the article "Adaptive Filters to improve Loudspeaker telephone" by South et al in Electronics Letters, Oct. 11, 1979, Vol. 15, no. 21, pages 673-674. According to a first known method, in order to reduce the electrical gain in the loop and thereby prevent the occurrence of oscillations, an attenuator is included in the loudspeaker channel or in the microphone channel depending on whether a speech signal appears in the microphone channel. Apart from a certain degree of complexity, said method renders the conversation unnatural, specifically because it is difficult to interrupt a speaker who is talking into the microphone. According to a second known method, which is based on echo-cancellation techniques, use is made of two adaptive filters which are controlled so as to form replicas of the electrical and acoustic paths of the loop in which the oscillations arise, the signals supplied by said filters being subtracted from the signals appearing in the two channels, Said second method has the disadvantage of a high complexity.
It is an object of the present invention to prevent the occurrence of oscillations in an electro-acoustic loop which comprises an acoustic transmitter and an acoustic receiver while mitigating the drawbacks of the known methods.